The present invention relates to the art of pressure molding objects of at least two different resins.
Pressure molding, particularly injection molding, of articles simultaneously made of two different resins is known and described in the literature. In that process several, but for parctical reasons usually only two, separate injection units are employed to supply molten plastic to a single gate, i.e. injection orifice, leading into a mold cavity. The plastics emerging from the two injection units are usually different. For example, one may be a solid resin while the other may contain a foaming agent and thus become porous in the mold; or, one may be a structurally superior material while the other may have the better barrier characteristics. Through appropriate valving one resin is injected into the mold cavity, whereby the volume so injected is less than sufficient to fill the mold cavity. Thus, this first plastic entering the mold cavity will not fill the same completely while advancing along the mold cavity walls which are chilled as usual. A chilled skin is thereby formed in contact with the mold cavity walls. Immediately following this step, the supply of this first resin is interrupted and the second resin is injected within the skin that has formed in contact with the mold wall. Alternately, the injection of the second resin is started and thereafter continued without interrupting the injection of the first resin. The supply of the second resin is continued until the cavity is completely filled by action of resin B and/or a foaming agent contained therein. When applied to the combination of a solid and a foamed resin, this process produces articles that have a solid, smooth, structurally resistant skin, filled with a porous material, resulting in strong and durable articles with low total plastic content and accordingly relatively low in coat. Examples of this molding technique are found extensively in the manufacture of furniture, instrument housings, automotive dash boards and other commonly used objects.
Another application, although less frequently encountered, is the combination of two resins, both solid: in other words, the "inner resin" does not contain a foaming agent. The purpose is to impart to the outer layer improved abrasion resistance or some other property which is desirable to have on the outside, and which is obtainable by the use of a relatively expensive resin; this is then followed by the injection of a less expensive resin which need not have the properties of the first one and is accordingly less costly.
In order to fill the mold cavity in the sequence indicated, it is necessary to provide suitable nozzles through which the sequential injection can be perfected. As an example, U.S. Pat. No. 4,376,625 shows a nozzle in which the first material enters through a central passage and second through an annular passage concentric with the first and in which a shuttle valve provides communication with the mold cavity, once for the first material and then for the second one as well. Such a procedure, however, is inconvenient and expensive, subject to breakdowns and inoperative when applied to multi-cavity molds.
A particular objective of the present invention is to extend the technique of multiple injection to multi-cavity molds without having to resort to complicated and wasteful cold runners. It was found to be impossible heretofore to apply the multiple injection technique to multi-cavity molds under employment of gating and molten plastic distribution principles that have proven to represent the most economical injection molding practice. This fact kept the multiple injection technique from being used for comparatively small objects, as for example, packages and containers which, in order to be molded economically, must be produced in multi-impression molds, the number of impressions often reaching 40 or more.
The device used in normal pressure molding practice for multi-impression molds is known as the hot runner, or insulated runner in which a heated manifold is used to distribute hot molten plastic from one injection source through a network of channels and through individual nozzles into each cavity of the multi-impression mold set. The hot runner, or insulated runner technique, is widely used. Reference can be made for example to U.S. Pat. Nos. 4,268,240, 4,213,751 and 4,333,629. All of these patents describe hot runners in which a temperature controlled manifold, usually contained within a plate, communicates with nozzles by way of suitable passages which in turn communicate with the mold cavity.
It is an objective of the present invention to render possible the use of a hot runner (or insulated runner) manifold distribution system for multi-component pressure molding. Problems stood in the way of accomplishing this objective heretofore. For example, clearly, upon completion of any given molding operation and before the molded part is removed from the injection mold, the hot runner manifold system including the nozzles contains stagnant resin which has last been injected. This resin residing in the manifold system would be the one to enter the mold cavity in the course of the subsequent shot while a like amount of resin would be furnished by the injection unit recharging the manifold system. The attempt to use a hot runner manifold system for multi-component injection has been impossible because the hot runner and the nozzles would remain charged, i.e. prefilled with the last injected resin so that upon opening communication between the manifold and the mold cavity the next time, that resin would be the first one to enter the mold, while in fact just the opposite should be accomplished. Accordingly, if a known and conventional hot runner system were to be used for multi-component injection, each first molded part would have one resin on the outside and the other on the inside while each next molded part would have the opposite sequence of layers, an obviously intolerable condition.
It is, therefore, a principal object of the present invention to develop a method and device for pressure molding objects of at least two different resins which method and device is convenient and expeditious to employ and which may be readily used on a commercial scale.
It is a further object of the present invention to provide a method and device as aforesaid which overcomes the disadvantages noted heretofore.
Further objects and advantages of the present invention will appear hereinbelow.